In recent years, dietary fibers have attracted notice as health foods. The dietary fibers are mainly composed of cellulose, hemicellulose, lignin, pectin and so forth. They are distinguished from so-called crude fibers conventionally available, and regarded as hardly digestible vegetable components contained in vegetable cell walls and inclusions contained in grains or the like. Hulls of grains or beans, including corn bran, wheat bran and rice bran, have attracted notice as sources of such dietary fibers. These are progressively acknowledged to have a correlation to an increase or decrease in serum cholesterol, prevention of corpulence and diabetes, appendicitis, and promotion of the elimination of poisonous substances in food. The hulls of grains or beans, however, have the disadvantages that they are not soluble in water unless treatment is applied and, when eaten, also give an unpleasant feeling even if they are finely powdered.
For these reasons, it has been attempted to extract hemicellulose from the hulls of grains or beans to obtain water-soluble dietary fibers. Hemicellulose can be extracted by subjecting the hulls of grains or beans to an alkali treatment. The hemicellulose thus extracted is found to exhibit inhibitory action against an increase in serum cholesterol (Japanese Patent Publication No. 1689/1984).
The hemicellulose obtained by alkali extraction, however, gives a considerably high viscosity when it is dissolved in water even in a small amount, and hence it has been disadvantageous in that when used, for example, in drink preparations it tends to give an unpleasant sensation when swallowing, and some difficulties are brought about also in preparation steps including filtration.
It was also attempted to obtain water-soluble dietary fibers by subjecting hulls of grains or beans to acid treatment or explosive crushing. These methods, however, result in hydrolysis of the hemicellulose to a monosaccharide or an oligosaccharide, and thus have the problem that the resulting products can no longer be said to be dietary fibers.